1. Field of the Invention
The present invention relates to an exhaust gas treatment apparatus for treating exhaust gases from internal combustion engines in which exhaust gas treatment efficiency is enhanced.
2. Description of the Related Art
Exhaust gases discharged from diesel engines contain pollutants, such as HC (hydrocarbons), CO (carbon monoxide), NOx (nitrogen oxides), and PM (particulate matter). Among these pollutants, NOx is difficult to purify using oxidation catalysts or three-way catalysts which are practically used for gasoline-fueled automobiles. As promising catalysts capable of purifying NOX, selective reduction type NOx catalysts (hereinafter referred to as “SCR catalysts”) are under development.
Examples of known SCR catalysts include catalysts which have a honeycomb structure including an active component, such as V, Cr, Mo, Mn, Fe, Ni, Cu, Ag, Au, Pd, Y, Ce, Nd, W, In, or Ir, carried on a carrier composed of TiO2, a binary oxide, such as SiO2—TiO2, WO3—TiO2, or SiO2—TiO2, or a ternary oxide, such as WO3—SiO2—TiO2 or MoO3—SiO2—TiO2, and in which NOx is reduced, in the presence of a reductant, such as ammonia, into nitrogen gas to perform purification.4NO+4NH3+O2→4N2+6H2O  (1)NO+NO2+2NH3→2N2+3H2O  (2)6NO2+8NH3→7N2+12H2O  (3)
In addition, a catalyst in which a support layer composed of particulates having catalytic activity, such as zeolite, is disposed on a monolith carrier is also known.
As the method of supplying ammonia as a reductant, a method is known in which urea is added from an aqueous urea tank to an exhaust system at a location upstream from an SCR catalyst thereby to generate ammonia. Urea is hydrolyzed by heat of exhaust gas or by a hydrolysis catalyst to generate ammonia. However, thermal decomposition of urea by heat of exhaust gas may generate high-melting-point materials, such as cyanuric acid, isocyanic acid, and melamine, resulting in a decrease in decomposition efficiency or a decrease in NOx reducing performance at the downstream location. Furthermore, these materials are known to be toxic.
Japanese Unexamined Patent Application Publication No. 2005-344597 (Patent Document 1) proposes that aqueous urea is vaporized for feeding, the fed urea is hydrolyzed to ammonia by a honeycomb, plate-shaped, or granular catalyst, and the resulting ammonia is used.
A method is also disclosed in which stored water is fed, under heating, to a cartridge in which urea in the solid state together with zeolite is stored, and urea is hydrolyzed to ammonia to be supplied (Japanese Unexamined Patent Application Publication No. 2002-89241, Patent Document 2).
Furthermore, a method is disclosed in which using a metal complex resin or urease enzyme as a catalyst, at a reaction temperature of 70° C. or lower, urea is converted to ammonium carbonate which is highly soluble in water and generates ammonia at a relatively low temperature, the resulting ammonium carbonate being used (Japanese Unexamined Patent Application Publication No. 2005-273509, Patent Document 3).
However, although generation of high-melting-point materials can be suppressed, when such a complex catalyst or enzyme catalyst is used even in the form of blocks, the catalyst is disintegrated into powder as operating time passes, which may result in clogging of pipes, and also the catalytic performance gradually degrades, which makes longtime stable use impossible.
On the other hand, many metal oxides have been used as heterogeneous catalysts, and used at high space velocity (SV) depending on the type of reaction. In such a case, in fixed bed reaction, most of the catalysts are used as molded bodies, such as granular, pellet-shaped, or honeycomb bodies. In the case where a catalyst is used in the form of granular or pellet-shaped molded bodies, when the size of the catalyst is small, the activity is relatively high, but a differential pressure may occur, for example, at high SV, resulting in difficulty in operation. When the size of the catalyst of the granular, pellet-shaped is large or the catalyst is honeycomb-shaped, the effectiveness factor decreases, resulting in insufficient performance. This necessitates an increase in the amount of the catalyst, use of a large reaction tower, or the like. In either case, disintegration into powder occurs when the catalyst is filled in or taken out of a reactor or the like, which is a problem Therefore, there has been a requirement for molded bodies which are excellent in terms of strength, abrasion resistance, etc.
With respect to such granular or pellet-shaped molded bodies, for example, a catalyst component is kneaded and subjected to extrusion molding, and the extrusion-molded material is cut into an appropriate length thereby to obtain pellet-shaped molded bodies. Before drying, the pellet-shaped molded bodies are granulated with a Marumerizer or the like to thereby obtain granular molded bodies.
Furthermore, in order to obtain honeycomb molded bodies, for example, a catalyst component is kneaded and subjected to extrusion-molding using multi-hole dies. However, distortion and deflection may occur, and cracks are easily generated during drying and firing, giving rise to a productivity problem. For this reason, it is difficult to obtain large honeycomb molded bodies. Another method is known in which a catalyst layer is formed on the surface of a honeycomb substrate composed of metal or ceramic. However, in this method, adhesion to the substrate is not very good.
In recent years, as in the case of fuel cells and the like, incorporation of a system including a reaction system into an apparatus has been under study. In such a case, in order to reduce the size of the apparatus and enhance the performance of the apparatus, membrane reactors have been actively developed.
For example, a fluorocarbon resin-containing porous body for a gas diffusion electrode has been disclosed in which a dispersion liquid, as a gas diffusion electrode material, containing fluorocarbon resin particulates, carbon black particulates, and as necessary, metal particulates selected from gold, silver, and platinum metals, and alloys of these metals, or metal oxide particulates thereof, is prepared, the pH of the dispersion liquid is adjusted, the zeta potential of the particulates is adjusted by addition of an ionically dissociative compound, and the gas diffusion electrode material is deposited on the surface of a conductive substrate, such as a wire gauze, by electrophoresis. That is, deposition of particulates on a conductive substrate by electrophoresis has been disclosed (refer to Japanese Unexamined Patent Application Publication No. 2002-121697, Patent Document 4).